The objectives of this project are to define segmental interactions between nociceptive and proprioceptive input originating from orofacial structures. Algesic stimulation of small- diameter muscle afferents excites in the spinal trigeminal nucleus and modulates proprioceptive responses. Altered jaw muscle spindles activity may disturb motor activity and kinesthetic sensibility. However, the neural basis for pain- induced modulation of craniofacial proprioception is not well understood. Three specific aims will address this issue. Aim 1. Define neural components responsible for the modulation of proprioceptive information in the presence of orofacial muscle pain. Three specific hypotheses are proposed. 1) Noxious chemical stimulation of the muscle produces differential modulation of responses from spindle primaries and secondaries to proprioceptive stimuli. 2) The caudle brainstem neurons that process small diameter muscle input are an integral link in the circuit for proprioceptive modulation. 3) The modulation of spindle afferents following noxious chemical stimulation of the muscle is mediated through the gamma fusimotor system. Aim 2. Investigate the effects of experimental muscle pain-induced modulation of spindle discharge on trigeminal alpha motoneurons. Intracellular recording techniques will be used to test whether changes in muscle spindle discharge induced by noxious chemical stimulation of the masseter muscle will cause transient changes in the excitability of masseter and synergist motoneurons during passive jaw stretch (hypothesis 4). Aim 3. Define the distribution and functional connectivity of the brainstem neurons that process jaw spindle signals, and determine their responses to noxious chemical stimulation of the craniofacial muscle. Two specific hypotheses are proposed. 5) Probst's tract collaterals from muscle spindle afferents project to functionally distinct populations of neurons throughout the rostrocaudal extent of the medical border of the spinal trigeminal nucleus and the PcRF. 6) The central proprioceptive neurons that receive input from jaw muscle spindle afferents at the level of Vi and Vc participate in coordinating changes in oral motility patterns in response to experimental muscle pain. The studies will contribute to our understanding of mechanisms underlying TMD, myofascial pain, and oromandibular dystonia.